In recent years, as digitization of video data or audio data evolves, a network which transmits computer data such as a control command and digital AV data typical of MPEG (Moving Picture Expert Group) together has been put into practical use at home or in cars. The demand for a variety of and flexible network connection forms goes on increasing, including increasing nodes and ordering of reliability for each group of apparatuses or for each data. In the past, for interconnection of networks of different topologies such as the bus type network and the loop type network, a gateway or a router was generally used in a network layer as described in a reference "Local Area Network" written by Naohiko Kamae, published by Shokodo, Jul. 6, 1992. FIG. 10 shows this example. In FIG. 10, reference numeral 101 designates a bus type network, 101a designates nodes connected to the bus type network 101, 102 designates a gateway or a router, 103 designates a loop type network, and 103a designates nodes connected to the loop type network 103. The bus type network 101 and the loop type network 103 are interconnected via the gateway or the router 102.
The loop type network comprises less optical fibers for connecting nodes and less optical modules, and therefore provides cost-effectiveness and high node extensibility. However, since breakage of parts of the optical fibers or failures of nodes causes the network to be in inoperable state and the failures are difficult to diagnose, the loop type network is less reliable and requires much maintenance. On the other hand, although the bus type network comprises more optical fibers or optical modules and thereby is not cost-effective, the breakage of parts of the optical fibers or the failures of the nodes causes the nodes to be in inoperable state but does not affect the whole network, and therefore it is reliable and requires little maintenance. For these reasons, use of a combination of these networks depending upon the use or importance of equipment can make up for drawbacks of respective networks, and thereby achieves a network which provides cost-effectiveness, high reliability, and high extensibility, and requires little maintenance. Since the gateway or the router is used for connecting networks, these apparatuses require packet switching at a high speed, and therefore require a computer system or complicated devices and thus they are not suitable for use at home or in cars from the point of view of the cost-effectiveness and scale.